Technical Field
The present invention relates to a method of evaluating the effectiveness of matrix acidizing treatment in a subterranean formation by injecting a plurality of salt solutions into multiple sections of the subterranean formation before an acidizing treatment, and collecting the salt solutions at the surface of the wellbore after producing the wellbore, followed by comparing the concentration of the injected salt solution to the collected salt solution.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Directional wells such as multilateral wells, shown in FIG. 1, and maximum reservoir contact (MRC) wells, are among the type of wells that are used nowadays to increase the production rate from an underground hydrocarbon reservoir. Well stimulation, along with the directional drilling technologies, is the key to maintain productivity of these types of wells. Reservoir characteristics, borehole stability, formation damage, zonal isolation, and hydraulic fracturing need to be considered during stimulation operations. Directional wells such as MRC and/or multilateral wells are usually drilled into low permeability reservoirs. Stimulation treatment is important for deciding whether directional wells can be successfully produced.
Formation damage that occurs during drilling may cause a significant drop in productivity of directional wells. Effective stimulation treatment is important to recover and enhance the performance of these wells. In most cases, a pre-completion clean-up step is carried out to reduce the formation damage.
Stimulation treatment in multi-lateral and MRC wells generally begins by removing the drilling filter cake from the sand face and ends with penetrating the invaded or damaged zone in the near wellbore area either by acid treatment or by hydraulic fracturing.
Two acid stimulation techniques are commonly used to treat multilateral wells. The first technique is tubing stimulation whereby tubing is run through the production tubing of a multilateral well to selectively treat individual laterals in the main reservoir. There are two challenges when coiled tubing is used to transport a stimulation fluid during stimulation treatment. The first is to accurately access the laterals in a multi-lateral well, and the second is to inject the stimulation fluid into the subterranean formation surrounding the target section without damaging the nearby permeable sections. Garzon et al. provided a method for successful stimulation of a target zone in a lateral well by using Fiberoptics enabled coiled tubing (FOECT). Real time distributed temperature survey (DTS) profiles were also shown to enable modification of the stimulation treatment [Garzon, F. O., Amorocho, J. R., Al-Harbi, M., Al-Shammari, N. S., Al-Ruwaished, A., Ayub, M., Noya, J. V. 2010, “Stimulating Khuff Gas Wells with Smart Fluid Placement”, Paper SPE 131917, SPE Deep Gas Conference and Exhibition, Manama, Bahrain, 24-26 Jan. 2010; incorporated herein by reference in its entirety]. The second acid stimulation technique involves pumping an acid solution from the surface of the well through the production tubing (i.e. bullhead stimulation treatments).
Although, acid stimulation was shown to be an effective treatment for directional wells, it was not the preferred treatment for tight reservoirs. Accordingly, acid fracturing was found to be the most effective stimulation treatment in directional wells in tight reservoirs [Joshi, S. D. 2003, “Cost/Benefits of Horizontal Wells”, Paper SPE 83621, SPE Western Regional/AAPG Pacific Section Joint Meeting, Long Beach, Calif. 19-24 May 2003; incorporated herein by reference in its entirety]. Acid fracturing refers to a stimulation treatment of a subterranean formation, wherein acid is pumped into the wellbore with a pressure above fracture pressure of the subterranean formation to cause hydraulic fractures in the formation. Acid also causes to etch the open faces of the induced fractures.
However, effective stimulation requires the acid to be diverted across all intermediate intervals to reach the low permeability sections. The use of diverting additives combined with stimulation treatments can increase the productivity of some multilateral wells, but in many cases this diversion technique is not effective in stimulating the desired number of laterals. For example, overstimulation of the heel section with untreated toe section is one of the challenges of stimulating long horizontal sections especially when reservoir pressure varies from the heel section to the toe section [SAU, R., Shuchart, C. E., & Grubert, M. A. 2014, “Advanced Completion and Stimulation Design Model for Maximum Reservoir Contact Wells”, Paper SPE 171800, Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, 10-13 Nov. 2014; incorporated herein by reference in its entirety].
One way of evaluating the effectiveness of stimulation treatments in a directional well is through using production logging. Production logs are often used to evaluate the effectiveness of stimulation treatments as well as for design and procedural modifications. To avoid the complexity of a stimulation fluid system that includes the use of acid diverters, and also to avoid the failure of acid diversion, multistage lateral stimulation is used by dividing each lateral into smaller segments and stimulating each segment individually.
One important aspect of stimulation treatments and production logs is the cost of these operations. Cost of stimulation treatment of directional wells such as MRC, multilateral, or horizontal wells increases proportionately with borehole length. For example, treatment of a well that has a total completion section length of about 10,000 ft requires at least 300,000 gallons of acid. The cost of such an operation is expected to be around 0.5-0.7 million dollars depending on the contract conditions, location of the well and prevailing prices [Aslam, J., & Al Salat, T. 1998, “Stimulation of Horizontal Wells in Carbonate Reservoirs”, Paper SPE 49493, Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, United Arab Emirates, 11-14 Nov. 1998; incorporated herein by reference in its entirety]. Therefore, the high cost of stimulation calls for more efficient and cost effective alternatives for stimulation treatments and production logs.
In view of the forgoing, one objective of the present invention is to provide an alternative method for production logging that evaluate the effectiveness of a stimulation treatment. The method is applicable to all different types of directional wells such as horizontal, extended reach, multi-lateral, and MRC wells.